Electrodeposition as a coating application method involves deposition of a film-forming composition onto a conductive substrate under the influence of an applied electrical potential. Electrodeposition has become increasingly important in the coatings industry because, by comparison with non-electrophoretic coating means, electrodeposition offers increased paint utilization, improved corrosion protection and low environmental contamination.
Initially, electrodeposition was conducted with the workpiece to be coated serving as the anode. This was familiarly referred to as anionic electrodeposition. However, in 1972 cationic electrodeposition was introduced commercially and has continued to gain in popularity. Today, cationic electrodeposition is by far the prevalent method of electrodeposition For example, a cationic primer coating is applied by electrodeposition to more that 80 percent of all motor vehicles produced throughout the world.
One important aspect of an electrodeposition coating system is its throw power. The term throw power refers to the ability to electrodeposit coatings in recessed areas of a work piece. A coating which has the ability to coat highly recessed areas is said to have high throw power. High throw power systems are desirable because a work piece can be more completely coated. For example, in automotive applications, coating of interior surfaces of double walled work pieces is desirable for increased corrosion resistance. Similarly, in the electrodeposition of other industrial articles, such as heaters or radiators having multiple walls or panels, high throw power electrodeposition systems are necessary to provide more corrosion resistance.
It is known, for a given system, that throw power can be increased by the application of higher voltage. However, excessively high voltage will cause dielectric breakdown and film defects commonly known as film ruptures. Thus, coatings which have a high rupture voltage are useful because higher throw power can be achieved while maintaining a smooth uniform film without ruptures. Throw power can also be affected by a higher conductivity of the electrodeposition bath. It is also generally recognized that higher molecular weight compositions tend to have higher throw power.
Resins having sulfonium salt groups have been used in cationic electrodepositable coating compositions to impart improved exposure durability and color stability as compared with their amine salt and quaternary ammonium salt counterparts. One disadvantage of sulfonium salt group-containing coating compositions, however, is their low throw power, due in large part to low bath conductivity.
The effect of low molecular weight amine salts on throwpower is known. U.S. Pat. No. 6,200,447 discloses the use of amine salt group-containing epoxy resins to improve throw power. Preference is given to low molecular weight amine salts, with the requirement that the amine salts be present in unbound form in the coating composition. U.S. Pat. No. 5,096,555 discloses a cationic electrodepositable coating composition comprising an amine salt or quaternary ammonium salt group-containing binder and a pre-dispersed cationic crosslinker. No mention is made of primary or secondary amines or their subsequently formed salts thereof, which have been found to give good conductivity and superior throw power.
U.S. Pat. No. 4,956,402 discloses a cationic electrodepositable coating composition that contains cationically charged urethane-containing resins, the urethane groups being covalently bonded to the backbone. The cationic charge on said binder is derived from basic amine groups. The invention is limited to compositions comprising epoxy resins having amine salt groups in the polymer backbone, which can be high in throw power without special modification. These compositions can be inherently self-crosslinking, and thus do not require a separate crosslinker.
Electrodepositable primer coating compositions, particularly those used in the automotive industry, typically are corrosion-resistant epoxy-based compositions crosslinked with aromatic isocyanates. If exposed to ultraviolet energy, such as sunlight, these compositions can undergo photodegradation. In some applications, a primer-surfacer is spray-applied directly to the cured electrodeposited coating prior to application of one or more top coats The primer-surfacer can provide a variety of properties to the coating system, including protection of the electrodeposited coating from photodegradation. Alternatively, one or more top coats can be applied directly to the cured electrodeposited coating and in such instances, these top coat(s) typically are formulated such that the top coat provides sufficient protection to prevent photodegradation of the electrodeposited primer coating. If the top coat(s) do not provide sufficient protection, photodegradation of the electrodeposited primer coating can result in delamination of the subsequently applied top coats from the cured electrodeposited primer coatings producing catastrophic failure of the cured coating system.
For example, if one or more top coats are sufficiently opaque to ultraviolet light transmission, such as by a high concentration of pigment and/or light absorbing compounds, little or no ultraviolet light can penetrate through the top coat(s) to the electrodeposited primer coating to cause photodegradation. However, if a thin top coat and/or a top coat which is not ultraviolet light absorbing is applied to the cured electrodeposited primer coating, ultraviolet light can pass through the top coat(s) resulting in photodegradation of the cured electrodeposited primer coating. Such a problem is likely to occur when a top coat is lightly pigmented with metal flake pigments which tend to allow transmission of ultraviolet light to the previously applied and cured electrodeposited primer coating.
A variety of approaches are known to avoid photodegradation of the cured electrodeposited coatings. As mentioned above, top coats can be formulated to have a high concentration of pigments which provide ultraviolet light opacity. Further, top coat formulations can include additives to prevent or diminish the transmission of ultraviolet light such as ultraviolet light absorbers (“UVAS”) and/or hindered amine light stabilizers (“HALS”) which can be used in combination with anti-oxidants, for example, phenolic antioxidants.
Other factors can exacerbate the photosensitivity of an epoxy-based primer, thereby contributing to delamination of a subsequently applied top coat from the primer coating. Such factors include, but are not limited to, the use of aromatic isocyanate crosslinkers, and overbake of the electrodeposited primer coating at excessive times and/or temperatures
U.S. Pat. No. 4,755,418 discloses a method of preventing the yellowing of the outermost coating of a multicoat coating system. The method comprises initially depositing onto a conductive substrate by cathodic electrodeposition a primer coating of at least one layer of an amine-epoxy resin adduct and a cross-linking agent; curing the primer to a hard, durable film; depositing a second coating onto the primer layer comprising at least one layer of a pigmented basecoat; depositing a third outermost coating onto the second coating comprising at least one layer of a clear top coat; and simultaneously curing the basecoat and the clear top coat. The electrodepositable primer coating composition contains a blocked polyisocyanate cross-linking agent selected from aliphatic polyisocyanates of at least six carbon atoms, the isocyanurates of aliphatic polyisocyanates, aromatic polyisoycanates having a molecular weight greater than 174, and the isocyanurates of aromatic diisocyanates having a molecular weight greater than 174.
U.S. Pat. No. 5,205,916 discloses electrodepositable primer compositions containing an aqueous dispersion of an epoxy-based ionic resin and an anitoxidant additive comprising a combination of a phenolic antioxidant and a sulfur-containing antioxidant. Such additives are disclosed as providing reduced overbake yellowing of the subsequently applied top coats as well as preventing intercoat delamination of these top coats upon exterior exposure.
U.S. Pat. No. 5,260,135 discloses photodegradation-resistant electrodepositable compositions comprising an epoxy-based ionic resin, a hindered amine light stabilizer present at levels of about 1 percent, and a phenolic antioxidant. Although effective for improving the resistance of the electrodeposited coating to photodegradation, the effect can vary somewhat due to the volatilization of the HALS present at the surface upon thermal curing of the composition. In some instances, the inclusion of HALS in electrodepositable coating compositions will provide only a marginal improvement for photodegradation resistance of the cured electrodeposited coating because the HALS can migrate into the subsequently applied top coating layers. Moreover, due to environmental and toxicity concerns, it is desirable to avoid the use of phenolic compounds such as the phenolic antioxidant mentioned above.
Although the aforementioned references disclose photodegradation resistant coating systems which can provide many advantages, each of the respective coating system disclosed therein can have one or more deficiencies, including low throw-power, excessive cost, toxicity issues, or marginal effectiveness. Accordingly, there remains a need in the coatings industry for a cost effective electrodepositable composition which retards photodegradation and delamination of subsequently applied top coats independent of the top coat composition(s). Additionally, there remains a need for a high throw power, photodegradation resistant electrodepositable composition that can be used in a single coat application.